Photothermographic elements for forming photographic images through heat development are disclosed, for example, in U.S. Pat. Nos. 3,152,904 and 3,457,075, D. Morgan and B. Shely, "Thermally Processed Silver Systems" in "Imaging Processes and Materials," Neblette, 8th Ed., Sturge, V. Walworth and A. Shepp Ed., page 2, 1969.
These photothermographic elements generally contain a reducible silver source (e.g., organic silver salt), a catalytic amount of a photocatalyst (e.g., silver halide), a toner for controlling the silver tone, and a reducing agent, typically dispersed in a binder matrix. Photothermographic elements are stable at room temperature. When they are heated at an elevated temperature (e.g., 80.degree. C. or higher) after exposure, redox reaction takes place between the reducible silver source (functioning as an oxidizing agent) and the reducing agent to form silver. This redox reaction is promoted by the catalysis of a latent image produced by exposure. Silver formed by reaction of the organic silver salt in exposed regions provides black images in contrast to unexposed regions, forming an image.
Such photothermographic materials have been used as microphotographic and medical photosensitive materials. However, only a few have been used as a graphic printing photosensitive material because the image quality is poor for the printing purpose as demonstrated by low maximum density (Dmax) and soft gradation.
With the recent advance of lasers and light-emitting diodes, scanners and image setters having an oscillation wavelength of 600 to 800 nm find widespread use. There is a strong desire to have a high contrast photosensitive material which has a high enough sensitivity and Dmax that it may comply with such output devices.
From the contemporary standpoints of environmental protection and space saving, it is strongly desired in the graphic printing field to reduce the quantity of spent solution. Needed in this regard is a technology relating to photothermographic materials for use in the graphic printing field which can be effectively exposed by means of laser image setters and produce clear black images having a high resolution and sharpness. These photothermographic materials offer to the customer a simple thermographic system which eliminates a need for solution type chemical agents and is not detrimental to the environment.
U.S. Pat. No. 3,667,958 discloses that a photothermographic element comprising a polyhydroxybenzene combined with a hydroxylamine, reductone or hydrazine has high image quality discrimination and resolution. This combination of reducing agents, however, was found to incur an increase of fog.
For producing a thermographic recording element having high Dmax and high contrast, it is effective to add to the element the hydrazine derivatives described in U.S. Pat. No. 5,496,695. Although this results in a thermographic recording element having high Dmax and high contrast, all of sensitivity, contrast, Dmax, Dmin, and storage stability of compounds are not fully satisfied.
Improvements in contrast and storage stability of compounds are achieved by using the hydrazine derivatives described in EP 762196A1, but the fully satisfactory level has not been reached.
Further, U.S. Pat. Nos. 5,545,515 and 5,635,339 disclose the use of acrylonitriles as the co-developer. With these acrylonitrile compounds, a fully satisfactory high contrast is not achieved, fog rises, and the photographic properties largely depend on the developing time.